Cameras are deceptively simple on the surface. Pick up your DSLR, look through the viewfinder, lock focus, and shoot. But there’s a lot more going on under the surface, including a process called Phase Detection Autofocus.
This phrase is often thrown around in the DSLR world, especially when buying a new camera. But many don’t actually know what this is or how it works!
What Is Autofocus?
Let’s start with the basics. There are two types of focus: auto and manual.
Manual focus is when the user has to manipulate the camera or twist the focusing ring in order to achieve focus.
Autofocus is an automatic process that does this all for you. It uses a computer to run a miniature motor that focuses the lens for you.
Focusing is the moving of the lens in and out until the sharpest possible image of the subject is projected onto the film. But to break it down into a more detailed explanation…
Contrast autofocus works by having the camera’s computer evaluate the histogram from the sensor. Then it moves the lens a smidge and reevaluates to see if there is more or less contrast.
If contrast increases, the camera continues moving the lens in that direction until contrast is maximized. If contrast decreased, the camera moves the lens in the other direction.
This process is repeated until there is high contrast. It does this because a perfectly focused image is one with the highest contrast.
With phase detection autofocus, when a point is in focus, the light rays coming from it will equally illuminate opposite sides of the lens. This is considered ‘in phase’, as the name suggests.
If the lens is focused in front of or behind the point in question, the light rays at the edge of the lens arrive in a different position. This is ‘not in phase’.
It’s not as complicated as it sounds.
How to Understand Phase Detection Autofocus
This is how phase detection autofocus works. An image hits a prism that then divides said image into two. If the image is in focus, the split images will line up. If not in focus, the images won’t line up.
The sensor knows which split image is which. That means it knows precisely in which direction and how much to move the focus motor to bring the split image together.
The light that passes through the sides of the lens is evaluated by two image sensors. Based on how the light reaches the image sensors, the phase detection autofocus system can determine if an object is front or back focused.
Since the phase detection system knows if an object is front focused or back focused, it can send exact instructions to the camera lens on which way to turn its focus and by how much.
This process is repeated until perfect focus is achieved. If focus cannot be achieved, the lens resets and starts ‘hunting’, or trying to find the focus.
Fun fact: Part of the reason that DSLR cameras are so heavy is because they have an actual prism inside of them to split the image onto the focus sensor. Mirrorless cameras are significantly smaller and lighter because they achieve the same result by doing this directly on the sensor.
Once perfect focus is achieved, the AF system sends a confirmation that the object is in focus. This is either the green square or the beep you hear, depending on the camera model. All of this complexity happens in a fraction of a second!
Phase detection autofocus is great for capturing movement because it is such a fast process.
When comparing camera models, you’ve likely heard about phase detection points. When a digital camera says it has a certain number of phase detection points, that means there are that many places on the sensor where it can compare the split image.
For each of these points, there are two tiny sensors. When the light reaches these two sensors, if an object is in focus, light rays from the sides of the lens converge right in the center of each sensor.
If both sensors have identical images on them, the object is in perfect focus. If an object is out of focus, the light would no longer converge and it would hit different sides of the sensor.
What Is Phase Detection Autofocus Best For?
Phase detection autofocus is very well suited for action photography. If your subject is moving phase detection will give you the fastest and most accurate autofocus.
Partner this autofocus system with with image tracking technology and multiple phase detection points, as well as AI Servo / Continuous Focus modes, and you have an equation for success.
Phase detection works just as well for other types of photography, such as portraits and still life. But action photographers will be most grateful for the inclusion of this system.
The advantages of phase detection autofocus include being incredibly fast and allowing the sensor to be used to assess the image’s depth of field. This gives an “electronic depth of field” preview.
You get an accurate idea of how the depth of field will look before a shot is even taken.
Knowledge is never something to regret – the more you know, the larger your tool kit!
By understanding how phase detection autofocus works, you can get rid of focusing issues, and know when something isn’t working right in your camera.
A note from Josh, ExpertPhotography's Photographer-In-Chief:
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